The properties which make human language a unique type of communication.

People use not only informative signals, which are unintentionally sent (e.g. sneezing: indicating cold), but also use language to intentionally communicate something.
There are six main features of human language which make it unique type of communication. What's more, these properties are unlikely to be found among communication systems of other creatures.
[To remember the six properties you may use DAPCUDD]
1. DISPLACEMENT - it enables users of language to describe events which not necessarily are happening at the present moment [like cat that meows], but may refer to past or future time. Thanks to this property, we can talk about things and places about which we are not sure and create fiction and possible future events. In contrast, it appears that animals use communication at this certain moment, here and now. If your cat is calling 'meow', you are likely to understand it as referring to that particular moment.
2. ARBITRARINESS – there is no 'normal' association between linguistic form and its meaning. The forms of human language do not fit the objects they describe and it is impossible to look at the word and from its shape determine its meaning.
However, there are some words like onomatopoeic ones which have sounds similar to real objects or activities (crash, cuckoo).
3. PRODUCTIVITY – human language in terms of potential utterance is infinite. It is still being enlarged with new expressions. In contrast to it, animals' systems of communicating are not flexible. Worker bees, who can communicate the place of nectar source taking advantage of set of signals relating to horizontal distance, fail to create new signals indicating vertical location.
4. CULTURE TRANSMISSION – it is a process of acquiring language in a cultural context. Humans do not have any predisposition to learn specific language [it is not given in genes] when they are born. They do not inherit language from parents, but from the surroundings and speakers with whom they have contact. However, a kitten irrespective of its surrounding will produce 'meow' rather instinctive and signals used in its communication will be also rather instinctive, not learnt.5. DISCRETNESS – the sounds which are used in language are discrete. Even though there is no big difference between sound 'p' and 'b' in utterance these two sounds are meaningful. Each sound in the language is treated as distinct.
6. DUALITY – [most economical device of human language, because with finite number of sounds we are able to produce infinite number of utterances] ''double articulation'': there are two levels at which language is organised simultaneously. At physical level, we are able to produce individual sounds e.g. 'p', 'b' and none of these individual sounds has any essential meaning. At the second level, we produce these sounds in a particular combination('bin', 'pin') where one word's meaning is different from the meaning of another word. Therefore, human language can produce many combinations of sounds with a distinct meaning due to limited set of distinct sounds.

[Sounds do not have meaning, the morph it the smallest meaningful unit of meaning]